The objective of the proposed study is to investigate the relatedness and the interaction of the genomes of murine leukemia virus (MuLV) and murine sarcoma virus (MuSV) using ts mutants of MuLV as probes to superinfect different cell lines which harbor defective MuSV. Particular emphasis will be placed on the mechanism of rescue of MuSV and the induction of transformation in these cell lines by MuLV. The following cell lines which are chronically infected with Moloney strain of MuSV are being used (1) 15F cells - a cell line which harbors defective MuSV but appears morphologically non-transformed. Upon infection of MuLV these cells become morphologically transformed and produce both MuSV and MuLV. (2) TB-349, a clone producing MSV in approximately 1000-fold excess over any detectable MuLV. TB or NRK cells freshly infected with TB-349 particles become transformed non-producers. (3) 6M2 cells - a clone of NRK cells chronically infected with a temperature sensitive mutant of MuSV derived from TB-349 cells. The ts mutant is defective with respect to the expression of transformation at the non-permissive temperature.